The present invention relates to a hydraulic amplifier, particularly to an amplifier suitable for use in conjunction with the power braking system of a motor vehicle, in which an amplifier piston is slidably arranged within an axial bore of a housing, the piston itself having a blind bore within which there is a control slide capable of being actuated in a suitable manner, as, for example, by the brake pedal of the vehicle. The hydraulic amplifier is provided with passages which place the face of the amplifier piston in communication with either the pressure conduit or the return conduit, depending on the axial position of the slide within the amplifier piston.
Amplifiers used in conjunction with the power braking system of a motor vehicle are normally of the so-called vacuum type. In the case of vehicles driven by an internal combustion engine, and especially gasoline-driven spark-ignition engines, such power brakes are operated off the engine intake manifold. If the vacuum prevailing in the engine intake manifold is not sufficient, a vacuum pump, or auxiliary vacuum pump, may be used for producing the necessary vacuum; this is frequently the case where the internal combustion engine is of the Diesel type, i.e., an engine burning Diesel fuel and operating without spark plugs on the compression ignition principle. One major drawback of such vacuum type amplifiers is that they are relatively bulky and occupy a good deal of space. Hence, various types of hydraulic amplifiers have been developed and used in recent times, in which a hydraulic medium under pressure is used for the purpose of obtaining the desired power braking effect. One such hydraulic amplifier is shown, for example, in ATZ (Automobiltechnische Zeitschrift or Automotive Technical Journal) 75 (1973) 7, pages 261 to 263, particularly FIG. 2a on page 262. One significant advantage of such a hydraulic amplifier is that it can be made relatively small, and it has been found particularly practical to employ this type of hydraulic amplifier when the vehicle in which it is to be used is one that must in any event be provided with a source of hydraulic fluid under pressure. This will be the case if the vehicle is equipped, for example, with power steering or with an automatic leveling system.
A significant drawback of the heretofore known hydraulic amplifiers of the above type is that they are expensive to manufacture, especially in mass-production quantities. The reason for this is that the amplifier piston and the control slide are provided with symmetrical control passages and thus form a pressure-equalized unit. This, in turn, makes it essential that the piston and slide be manufactured to very close tolerances in order, firstly, to allow the slide to move readily within the axial bore of the piston and, secondly, to avoid leakage of the hydraulic medium which, in turn, would cause the pressure in the hydraulic pressure source to drop. To prevent such leakage, a coupling or disconnect valve is normally interposed in the conduit leading from the hydraulic amplifier to the source of fluid medium under pressure, this valve being closed whenever the brake is not actuated so as to hydraulically separate the piston-and-slide unit from the source of hydraulic pressure and/or the pressure reservoir or sump. This, in turn, requires the provision of additional structural means, such as an inclined tract carried by the amplifier piston, for opening the associated valve or valves whenever the brake is actuated.
It is, therefore, the primary object of the present invention to provide an hydraulic amplifier of the above type which avoids and overcomes the mentioned drawbacks, while functioning in a fully satisfactory manner.